Abstract: A fastening device comprising a block of plastic material of square or alternative cross section. The block has a threaded bore for the receipt of a bolt which completes the fastening, and additionally, the block has a through slot extending from one end making the block of a U-shape so that it has legs and a base. This configuration is such that the legs can be sprung together so that they can be pushed into the end of a square sectioned cable ladder tube to be connected at right angles to a cable ladder rail, whereupon the legs will spring apart so that a projection on the legs engage in apertures in the tube walls and the block cannot be retracted. To secure the rail to the tube end, the bolt is passed through a hole in the rail and is screwed into the block, which is locked in the tube end. The advantages are pre-insertion of the blocks in the factory and simple assembly of the cable ladder on site, and secure connection joining of the rail and tube ends.